Tooth interference in an external involute spur gear pair refers to the phenomenon where the gear teeth come into contact or interfere with each other during operation. This interference can lead to undesirable effects such as noise, vibration, increased wear, and reduced efficiency. The interference primarily occurs at the tooth tip and tooth root regions.

There are several ways to reduce tooth interference in a gear pair, and the correct answer is option 'D', which states that increasing the number of gear teeth can help in reducing tooth interference. Let's understand why this is the correct answer in detail:

1. Increasing the number of gear teeth:
- When the number of gear teeth is increased, the size of each tooth decreases, resulting in reduced tooth thickness.
- The reduced tooth thickness helps in minimizing the possibility of tooth interference, especially at the tooth tip and tooth root regions.
- By increasing the number of gear teeth, the contact ratio between the mating gears also increases. A higher contact ratio ensures smoother gear meshing and reduces the likelihood of tooth interference.

Other options explained:
2. Decreasing center distance between gear pair:
- Decreasing the center distance between the gear pair does not directly address tooth interference.
- Center distance primarily affects the gear meshing characteristics, such as the gear ratio and the contact ratio, but it does not directly influence tooth interference.

3. Decreasing module:
- The module is the ratio of the pitch diameter to the number of teeth in a gear.
- Decreasing the module results in smaller gear teeth and reduces the tooth thickness.
- However, decreasing the module is not the most effective solution for reducing tooth interference because it may lead to other design constraints such as reduced gear strength and increased sensitivity to manufacturing errors.

4. Decreasing pressure angle:
- The pressure angle is the angle between the line of action and the common tangent to the pitch circles of mating gears.
- Decreasing the pressure angle can help in reducing the contact stress and improving load distribution.
- However, it does not directly address tooth interference and may have other design implications.

In summary, increasing the number of gear teeth in an external involute spur gear pair is an effective way to reduce tooth interference. This approach ensures smaller tooth sizes, reduces tooth thickness, and increases the contact ratio, resulting in smoother gear meshing and improved performance.